There are a number of types of polyolefin pipes available in the market. There are concerns about the durability of the pipes, and particularly gas pipes over the long term. One concern is the lifetime of the pipe under pressure typically from about 400 to 800 psi. One approach has been to try to reduce the crystal size in polyolefin pipes. While crystal size may be decreased in other polymers using nucleating agents the crystal size in polyethylene does not seem to be significantly affected by the presence of synthetic nucleating agents. As a result it is difficult to obtain the combinations of stiffness and toughness in polyolefin pipes (e.g. being tough but flexible as opposed to brittle) to obtain an increase in the long term hydrostatic strength of polyolefin gas pipes.
U.S. Pat. No. 5,198,484 issued Mar. 30, 1993 to Michael J. Mannion, assigned to Milliken Research Corporation discloses the use of derivatives of sorbitol and xylitol as clarifying agents in polyolefins and in particular polyethylene and polypropylene. The patent teaches that the bubbles may be reduced in products fabricated from polyolefins containing such agents by dissolving the "clarifying" agent in the polyolefin at a temperature of at least 170.degree. C. However, the patent does not suggest any particular applications in which the polyolefins containing such "clarifying" agents are useful.
The present invention seeks to improve the long term hydrostatic strength of polyolefin pipes, such as gas pipes by incorporating such "clarifying" agents into the polyolefins used to manufacture such pipes.